Gametogenesis: Spermatogenesis and Oogenesis Notes | Study Biology Class 12 - NEET

NEET: Gametogenesis: Spermatogenesis and Oogenesis Notes | Study Biology Class 12 - NEET

The document Gametogenesis: Spermatogenesis and Oogenesis Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
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1. Formation of Gametes

The primary sex organs – the testis in males and the ovaries in females – produce gametes, i.e., sperms and eggs, through a process known as gametogenesis.

  • GnRH, FSH, and LH all play a role in gametogenesis. 
  • Aside from this hormone, vitamin E is also required for gametogenesis. 
  • Sterility is caused by a vitamin E deficiency. 
  • Vitamin A is also necessary for the development of healthy gametes. 

 Ovum (female gamete) & Sperm (male gamete) Ovum (female gamete) & Sperm (male gamete)

  • The stages of gametogenesis are as follows:
    (i) Multiplication Phase
    (ii) Growth Phase
    (iii) Maturation Phase
  • Because there are two kinds of gametes, spermatozoa and ova.

Try yourself:The process of formation of haploid male and female gamete is known as which of following
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  • Gametogenesis can be divided into two categories: Spermatogenesis and Oogenesis.
  • The formation of spermatozoa is referred to as spermatogenesis, whereas the formation of ova is referred to as oogenesis. 
  • Both spermatozoa and ova are derived from extra-gonadal primordial germ cells or PGCs. 
  • Spermatogenesis occurs in the testicular seminiferous tubules and oogenesis occurs in the ovarian follicles. Gamete formation begins during puberty.

2. Spermatogenesis

  • The process of formation of spermatozoa in the seminiferous tubules of the testis is known as spermatogenesis. The wall of seminiferous tubules is lined by cuboidal spermatogonia (male germ cells) and columnar Sertoli cells.
  • Spermatogonia are derived from primordial germ cells.
  • At puberty, spermatogonia divide to form spermatozoa by the process called as spermiogeneis and after spermiogenesis, sperm heads become embedded in the Sertoli cells. Sertoli cells provide nourishment to developing sperms.
  • Sertoli cells form the 'blood testis barrier and protect the sperm from the immune system of the body.
  • Sertoli cells function as endocrine glands. i.e. secrete three types of biochemicals:
    (i) Antimullerian hormone: it inhibits Mullerian duct in males.
    (ii) Inhibin hormone: It gives negative feedback to the pituitary gland (mainly) and hypothalamus.
    (iii) Androgen Binding Protein: It concentrates testosterone in seminiferous tubules to aid spermatogenesis.

Spermatozoa are formed in the wall of the seminiferous tubules of the testes:

  • The spermatogonia (type A) or germ cells (44 + X Y) divide mitotically, to give rise to more spermatogonia of type A and also spermatogonia of type B.
  • The spermatogonia (type B) (44 + XY) enlarge, to form primary spermatocytes.
  • The primary spermatocytes (44 + XY) now divide so that each of them forms two secondary spermatocytes. This is the first meiotic division: it reduces the number of chromosomes to half i.e 23.
  • Each secondary spermatocyte has 22 + X or 22 + Y chromosomes. It divides to form two spermatids. This is the second meiotic division and this time there is no reduction in chromosome number(spermatocytogenesis). 
  • Each spermatid (22 + X or 22+Y) gradually changes its shape to become a spermatozoon. This process of transformation of a circular spermatid to a spermatozoon is called spermiogenesis/spermateleosis
  • In human beings the maturation phase is the longest phase of spermatogenesis.

Difference between Spermatogenesis & SpermiogenesisDifference between Spermatogenesis & Spermiogenesis

Try yourself:Diploid primary germ cells in male is
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Hormonal Control of Spermatogenesis 

  • Spermatogenesis is initiated due to an increase in gonadotropin-releasing hormone (GnRH) by the hypothalamus. 
  • GnRH acts on the anterior lobe of the pituitary gland to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH). 
  • LH acts on the Leydig's cells of the testes to secrete testosterone
  • FSH acts on Sertoli cells of the seminiferous tubules of the testes to secrete an androgen binding protein (ABP) and inhibin. 
  • ABP concentrates testosterone in the seminiferous tubules. Inhibin suppresses FSH synthesis.
  • FSH acts on spermatogonia to stimulate sperm production.

Try yourself:The Leydig cells are a source of
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Hormonal Control of SpermatogenesisHormonal Control of Spermatogenesis

Structure of Sperm

Around 200-300 million sperms are ejaculated at once.Gametogenesis: Spermatogenesis and Oogenesis Notes | Study Biology Class 12 - NEETStructure of Sperm

  • The Head contains acrosome apically, which contains enzymes that facilitate the entry of sperm into the ovum. It is followed by an elongated nucleus (haploid).
  • The middle piece has multiple mitochondria, that provide energy for the movement of sperms.
  • The tail is a flagellum that protrudes out of the cell body and is responsible for the vigorous motility of sperms. The tail helps sperm in swimming so that they can reach the ovum.
  • Secretions from the epididymis, vas deferens, seminal vesicle, and prostate are required for sperm maturation and motility.

Try yourself:Match between the following representing parts of the sperm and their functions and choose the correct option.

Gametogenesis: Spermatogenesis and Oogenesis Notes | Study Biology Class 12 - NEET

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2. Oogenesis

  • The process of formation of a mature female gamete is called Oogenesis.
  • Like spermatogenesis oogenesis process also can be divided into three stages :
    (A) Multiplication Phase
    (B) Growth Phase
    (C) Maturation Phase

Phases of OogenesisPhases of Oogenesis

(A) Multiplication Phase

  • During the embryonic development stage primordial germ cells or gamete mother cells repeatedly divide by mitosis to form a large number of diploid oogonia in each foetal ovary, no more oogonia are formed and added after birth.
  • This process completes in the embryo stage of females in most higher animals.

(B) Growth Phase

  • Like spermatogenesis, in this process oogonia grows in size and forms primary oocytes.
  • The growth phase is the longest phase in oogenesis in oviparous animals.
  • During the growth phase the size of the egg increases many times.

(C) Maturation Phase

  • It is the longest phase in humans.
  • In contrast to males the initial steps in egg production occur prior to birth. By the time the foetus is 25 weeks old, all the oogonia that she will ever produce are already formed by mitosis. These diploid cells develop into primary oocytes, begin the first steps of the first meiotic division, proceed up to diplotene (prophase-I) and then stop any further development. The oocytes grow much larger and complete meiosis I, forming a large secondary oocyte and a small polar body that receives a very little amount of cytoplasm but one full set of chromosomes.
  • First polar body may undergo degeneration due to a lack of cytoplasm or may divide.
  • Whereas the secondary oocyte proceeds as far as the metaphase stage of meiosis II. However, it then stops advancing any further, it awaits the arrival of the spermatozoa for completion of the second meiotic division.
  • Entry of the sperm restarts the cell cycle breaking down MPF (Metaphase promoting factor and turning on the APC (Anaphase promoting complex). Completion of meiosis II converts the secondary oocyte into a fertilised egg or zygote (and also a second polar body)

Try yourself:which one is longer one duration
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Hormonal Control of Oogenesis 

  • GnRH secreted by the hypothalamus stimulates the anterior lobe of the pituitary gland to secrete LH and FSH. 
  • FSH stimulates the growth of Graafian follicles and also the development of egg/oocyte within the follicle to complete the meiosis I to form a secondary oocyte.
  • FSH also stimulates the formation of estrogens. 
  • LH induces the rupture of the mature Graafian follicle and thereby the release of secondary oocytes. Thus LH causes ovulation. 
  • In brief ovulation in human beings may be defined as the release of the secondary oocyte from the Graafian follicle. 
  • The remaining part of the Graafian follicle is stimulated by LH to develop into corpus luteum ("yellow body").
  • The rising level of progesterone inhibits the release of GnRH, which in turn, inhibits the production of FSH, LH and progesterone.

Hormonal Control of OogenesisHormonal Control of Oogenesis

Ovulation

  • In humans, the ovum is released from the ovary in the secondary oocyte stage. 
  • Thus in human beings, ovulation is the release of the secondary oocyte from the ovary. 

Process of OvulationProcess of Ovulation

  • The wall of the ovary gets ruptured to release the oocyte. 
  • In humans, ovulation occurs about 14 days before the onset of the next menstruation. Ovulation is induced by LH. 
  • The maturation of the ovum is completed in the mother's Fallopian tube usually after the sperm has entered the secondary oocyte during fertilization.

Try yourself:Mature Graafian follicle is generally present in the ovary of a healthy human female around.
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Structure of Ovum

  • The mature ovum or a female gamete is spherical in shape. 

An OvumAn Ovum

  • The human ovum is almost free of yolk and is said to be alecithal. 
  • Its cytoplasm is called ooplasm containing a large nucleus, termed the germinal vesicle. 
  • The nucleus contains a prominent nucleolus. 
  • There are no centrioles in the ovum. 
  • The cytoplasm is enveloped by the plasma membrane (cell membrane). 
  • Very small vesicles called cortical granules are present under the plasma membrane.
  • A narrow perivitelline space is present outside the plasma membrane. 
  • Just outer to perivitelline space, there is thick, acellular zona pellucida; probably secreted by the follicular cells.
  • Outer to the zona pellucida there is very thick cellular corona radiata.
  • The latter is formed of radially elongated follicular cells.
  • The side of the ovum which extrudes polar bodies is termed the animal pole. 
  • The opposite side is called a vegetal pole. The human ovum (egg) loses its ability to be fertilized about 24 hours after ovulation.
  • In human beings, an ovum is released from the ovary as a secondary oocyte. 

Significance of Oogenesis

  • One oogonium produces one ovum and three polar bodies. 
  • Polar bodies have a small amount of cytoplasm. It helps to retain a sufficient amount of cytoplasm in the ovum which is essential for the development of the early embryo. The formation. of polar bodies maintains half the number of chromosomes in the ovum. 
  • During meiosis first crossing over takes place which brings about variation. 
  • Oogenesis occurs in various organisms. therefore, it supports the evidence of basic relationships among the organisms.

Difference Between Male Sperm & Female Egg

Gametogenesis: Spermatogenesis and Oogenesis Notes | Study Biology Class 12 - NEET

Similarities in Spermatogenesis and Oogenesis

  1. Both processes consist of three main phases, viz., multiplication, growth and maturation phases.
  2. In the multiplication phase, the primordial germ cells of testes and ovaries proliferate mitotically, forming numerous gametogonia (spermatogonia/oogonia) in both processes. 
  3. In the growth phase, the cells accumulate food reserves and grow to primary gametocytes (spermatocytes/oocytes) in both processes.
  4. Maturation phase in both processes comprises two successive divisions, first meiotic and second meiotic, resulting in the formation of secondary gametocytes and gametes respectively.

Differences in Spermatogenesis and Oogenesis

Gametogenesis: Spermatogenesis and Oogenesis Notes | Study Biology Class 12 - NEET

Try yourself:Which of these is a major difference between oogenesis and spermatogenesis?
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The document Gametogenesis: Spermatogenesis and Oogenesis Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
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